1. Field of the Invention
The present invention relates, in general, to a diffraction-type light modulator and, more particularly, to a fishbone diffraction-type light modulator, in which a lower micromirror is provided on a silicone substrate, and an upper micromirror is spaced apart from the silicone substrate and has a plurality of openings through both sides thereof, so that the upper micromirror and the lower micromirror form pixels.
2. Description of the Prior Art
Generally, optical signal processing technology has advantages in that a great amount of data is quickly processed in a parallel manner unlike a conventional digital information processing technology in which it is impossible to process a large amount of data in real-time, and studies have been conducted on the design and production of a binary phase only filter, an optical logic gate, a light amplifier, an image processing technique, an optical device, and a light modulator using a spatial light modulation theory.
Of them, the spatial light modulator is applied to optical memory, optical display, printer, optical interconnection, and hologram fields, and studies have been conducted to develop displays employing it.
The spatial light modulator is embodied by a reflective deformable grating light modulator 10 as shown in FIG. 1. The modulator 10 is disclosed in U.S. Pat. No. 5,311,360 by Bloom et al. The modulator 10 includes a plurality of reflective deformable ribbons 18, which have reflective surface parts, are suspended on an upper part of a substrate 16, and are spaced apart from each other at regular intervals. An insulating layer 11 is deposited on the silicon substrate 16. Subsequently, a sacrificial silicon dioxide film 12 and a silicon nitride film 14 are deposited.
The nitride film 14 is patterned by the ribbons 18, and a portion of the silicon dioxide film 12 is etched, thereby maintaining the ribbons 18 on an oxide spacer layer 12 using a nitride frame 20.
In order to modulate light having a single wavelength of λo, the modulator is designed so that thicknesses of the ribbon 18 and oxide spacer 12 are each λo/4.
Limited by the vertical distance (d) between the reflective surface 22 of each ribbon 18 and the reflective surface of the substrate 16, the grating amplitude of the modulator 10 is controlled by applying a voltage between the ribbon 18 (the reflective surface 22 of the ribbon 18 acting as a first electrode) and the substrate 16 (a conductive layer 24 of a lower side of the substrate 16 acting as a second electrode).
In its undeformed state, with no voltage applied, the grating amplitude is λo/2, and the total round-trip path difference between light beams reflected from the ribbon and substrate is one wavelength λo, and thus, the phase of the reflected light is reinforced.
Accordingly, in its undeformed state, the modulator 10 acts as a plane mirror when it reflects light. In FIG. 2, reference numeral 20 denotes incident light and reflected light in its undeformed state.
When a proper voltage is applied between the ribbon 18 and substrate 16, the electrostatic force enables the ribbon 18 to be moved downward toward a surface of the substrate 16. At this time, the grating amplitude is changed to λo/4. The total round-trip path difference is a half of a wavelength, and light reflected from the deformed ribbon 18 and light reflected from the substrate 16 are subjected to destructive interference.
The modulator diffracts incident light 26 resulting from the interference. In FIG. 3, reference numerals 28 and 30 denote light beams diffracted in a +/− diffractive mode (D+1, D−1) in a deformed state.
However, the Bloom's light modulator adopts an electrostatic method to control the position of the micromirror, which has disadvantages in that the operating voltage is relatively high (usually, 30 V or so) and the correlation between the applied voltage and the displacement is not linear, resulting in unreliable light control.
A light modulator disclosed in the patent of Bloom may be used as a device for displaying images. In this regard, at least two adjacent elements may form one pixel. Needless to say, three, four, or six elements may form one pixel.
However, the light modulator according to the patent of Bloom has a limit of miniaturization. In other words, it is impossible to reduce the width of the element of the light modulator to 3 μm or less, or to reduce an interval between the elements to 0.5 μm or less.
Furthermore, the formation of diffraction pixels using the elements requires at least two elements, thus the light modulator has a limit of miniaturization of the device.